Bioalcohol, such as methanol, ethanol, butanol, propanol, etc., may be derived from biological materials, i.e., biomass primarily through fermentation. Such production can proceeding by typical chemical processing, as is used with natural gas, or by fermentation of sugars. Prior art bioalcohols may be derived from a number of sources, many of which are time consuming and/or cost intensive to produce or manufacture. The prior art processes for producing bioalcohols would benefit greatly from an improved and more efficient method of producing alcohol.
Existing technologies in processing industries are similar in concept in that they all require an input of energy to produce a final product. For example, some technologies include a pressurized homogenizer, which uses a sequential valve assembly to increase fluid pressure in the material being processed. Such a device requires a large energy input, producing a high outlet pressure, usually in excess of 5,000 psi.
Cavitation is defined as the generation, subsequent growth and ultimate collapse of vapor- or gas-filled cavities in liquids resulting in significant energy concentration and release on extremely small temporal and spatial scales. As understood in this broad sense, cavitation includes the familiar phenomenon of bubble formation when water is brought to a boil under constant pressure. In engineering and science, the term cavitation is used to describe the formation of vapor-filled cavities in the interior or on the solid boundaries created by a localized pressure reduction produced by the dynamic action of a liquid system.
Hydrodynamic cavitation is essentially generated by a change in bulk pressure in a liquid flow by variation of the velocity of the flow through well-defined geometries. In the simplest situation, hydrodynamic cavitation can be generated by forcing or throttling high pressure discharge from a pump through constrictions such as a venturi or an orifice. In this case, the velocity of the flow increases with reducing flow area causing a concurrent reduction in bulk pressure. If the throttling is sufficient, the pressure in the flow in the region downstream of the constriction may actually fall to or even below the vapor pressure of the medium. This causes the release of dissolved gas in the medium or generation of vapor bubbles in the liquid medium. These bubbles undergo oscillation with a recovery of pressure in the region further downstream resulting in a final transient collapse. The oscillations of the bubbles generate intense microturbulence in the medium causing vigorous mixing.
For a heterogeneous reaction system, this turbulence can create a fine emulsion between phases generating high interfacial area that can enhance the reaction kinetics. At the transient collapse of the bubble, the temperature and pressure in the bubble can reach extremely high values (˜3000 K, ˜100 bar or even higher) that can cause decomposition of the solvent vapor entrapped in the bubble resulting in generation of extremely reactive radicals that can accelerate the kinetics of a chemical reaction. The amplitude of the radial oscillation of the cavitation bubble and the intensity of collapse depends on the extent of variation in bulk pressure (or the bulk pressure gradient), which is characterized by a cavitation number. For a cavitation number equal to or less than 1, the bulk pressure gradient is high enough to cause transient cavitation. As the cavitation number increases above 1, the intensity of radial motion of the cavitation bubbles reduces. The cavitation bubbles experience small amplitude oscillatory motion, which can give rise to intense microturbulence in its vicinity.
Cavitation can occur at numerous locations in a fluid body simultaneously and can generate very high localized pressure and temperature on extremely small time scales, e.g., dozens of nanoseconds. Cavitation also results in the generation of localized turbulence and liquid micro-circulation, enhancing mass transfer—which is a prominent effect, especially for heterogeneous (either liquid-liquid or solid-liquid) systems. Thus, mass transfer-limited reactions, endothermic reactions and reactions requiring extreme conditions can be effectively carried out using cavitation. Moreover, radicals generated during cavitation due to the homolytic dissociation of the bonds of molecules trapped in the cavitating bubbles or in the affected surrounding liquid, result in the occurrence of certain reactions.
The flow essentially undergoes a sudden contraction and expansion that generates essential pressure variation for the in-situ generation and collapse of either vapor or gas bubbles. As stated earlier, these bubbles undergo volume oscillations and a transient collapse, which can create cavitation effects by intense energy concentration that results in extremes of temperature and pressure and also intense convection due to micro-turbulence and shock waves. However, this effect is seen either inside the bubble (of initial size ˜50-100 microns, which is compressed to about 1/10th of its initial size) or in the bulk liquid in close proximity to the bubble. Thus, the energy concentration created by transient bubbles is on an extremely small time and temporal scale.
Through these contractions and expansions, the flow may get separated from the walls of the conduit for a high Reynolds number. In this case, there is significant loss in the pressure head of the flow, which is manifested in terms of generation of turbulence in the flow. The turbulence creates fluctuations in the bulk pressure at low frequencies (1 to 2 kHz). These turbulent fluctuations are essentially superimposed over the mean pressure of the flow that keeps on increasing with the expansion of the flow. These fluctuations alter the behavior or pattern of radial motion of the cavitation bubble. In this case, the bubble undergoes an explosive growth followed by a transient implosive collapse. The cavitation effect produced by these bubbles is several folds higher than the bubbles in simple venturies or converging-diverging nozzles, where such flow separation does not occur. The difference in the cavitation bubble behavior in a orifice flow and in a venturi flow has been studied at length. (VS Moholkar and AB Pandit, Chemical Engineering Science, 2001).
In homogenous reactions, both the reagents and products remain in the same phase. The mechanical or physical effects of cavitation (e.g., generation of high intensity micro-turbulence) play a smaller part in such reactions in comparison with the chemical effects of creation of high-energy intermediates. In heterogeneous reactions, cavitation bubbles collapsing at or near the phasic interface undergo asymmetric collapse, giving rise to high velocity liquid microjets (with velocities in the range of 100-150 m/s). These microjets can give rise to several effects such as erosion of the surface or fragmentation and size reduction of the particles. Due to these effects, surface area available for the reaction between the phases is significantly increased, thus improving the rate of reaction. In case of catalytic reactions, microjets assist desorption of products from the catalyst surface, which helps in keeping the catalyst surface ‘fresh’ for reaction. Microjets also assist desorption of the catalyst poisons attached to the catalyst surface that helps in cleaning of the catalyst. Moreover, adsorption/desorption of the reactants/products on the catalyst surface is also facilitated by the microturbulence generated by cavitation bubbles.
TABLE 1 Comparison of energy efficiency for different methods.Time,Yield/energy,MethodminYield, %kJ−1Acoustic10998.6 × 10−5Conventional with stirring180982.7 × 10−5Presented flow-through899.92.6 × 10−3
It can be seen from Table 1 that reactions that take place in a flow-through cavitation generator are correspondingly about 30 times and 100 times more efficient compared to acoustic cavitation the agitation/heating/refluxing method.
Accordingly, there is a need for a method to carry out heterogeneous reactions that does not require a large amount of energy input. Further, there is a need for such a method that avoids potentially dangerous, high-pressure operation. Furthermore, there is a need for an improve method of producing alcohol from biomass that is more efficient and more cost effective. The present invention fulfills these needs and provides further related advantages through the utilization of hydrodynamic flow-through cavitation and the chemical and physical reactions and process involved.